Reorganize _qt module

[tlambert03]

the single file was getting a bit long. Splitting into different files.
This also allows /_qt/__init__.py to start to reveal what the backend "API" is... and what we would need to implement to add a new widget backend.

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 0.14%.
The diff coverage is 96.21%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   95.61%   95.76%   +0.14%     
==========================================
  Files           4       13       +9     
  Lines         890      921      +31     
==========================================
+ Hits          851      882      +31     
  Misses         39       39              

Impacted Files	Coverage Δ
magicgui/_qt/widgets/file_dialog.py	93.84% <93.84%> (ø)
magicgui/_qt/_qt.py	95.12% <95.12%> (ø)
magicgui/_qt/constants.py	95.45% <95.45%> (ø)
magicgui/_qt/__init__.py	100.00% <100.00%> (ø)
magicgui/_qt/categorical.py	100.00% <100.00%> (ø)
magicgui/_qt/types.py	100.00% <100.00%> (ø)
magicgui/_qt/widgets/__init__.py	100.00% <100.00%> (ø)
magicgui/_qt/widgets/data_combobox.py	100.00% <100.00%> (ø)
magicgui/_qt/widgets/double_slider.py	100.00% <100.00%> (ø)
magicgui/_qt/widgets/widget.py	100.00% <100.00%> (ø)
... and 13 more

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